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Abstract:

The present invention relates to a nitrogen oxide storage catalyst
comprising: a substrate; a first washcoat layer disposed on the
substrate, the first washcoat layer comprising metal oxide support
particles and a nitrogen oxide storage material comprising at least one
metal compound selected from the group consisting of alkaline earth metal
compounds, alkali metal compounds, rare earth metal compounds, and
mixtures thereof, at least a portion of said at least one metal compound
being supported on the metal oxide support particles; and a second
washcoat layer disposed over the first washcoat layer, said second
washcoat layer comprising Rh, wherein the first washcoat layer contains
substantially no Rh, and wherein the second washcoat layer is disposed on
100-x % of the surface of the first washcoat layer, x ranging from 20 to
80.

Claims:

1. A nitrogen oxide storage catalyst comprising: a substrate; a first
washcoat layer disposed on the substrate, the first washcoat layer
comprising metal oxide support particles and a nitrogen oxide storage
material comprising at least one metal compound selected from the group
consisting of alkaline earth metal compounds, alkali metal compounds,
rare earth metal compounds, and mixtures thereof, at least a portion of
said at least one metal compound being supported on the metal oxide
support particles; and a second washcoat layer disposed over the first
washcoat layer, said second washcoat layer comprising Rh, wherein the
first washcoat layer contains substantially no Rh, and wherein the second
washcoat layer is disposed on 100-x % of the surface of the first
washcoat layer, x ranging from 20 to 80.

2. The nitrogen oxide storage catalyst according to claim 1, wherein the
first and second washcoat layers are both disposed on the substrate
starting from one end and/or side of the substrate body.

3. The nitrogen oxide storage catalyst according to claim 1, wherein the
second washcoat layer further comprises metal oxide support particles on
which Rh is at least partially supported.

4. The nitrogen oxide storage catalyst according to claim 1, wherein the
metal oxide support particles comprise at least one metal oxide selected
from the group consisting of alumina, zirconia, zirconia-alumina,
baria-alumina, lanthana-alumina, lanthana-zirconia-alumina, and mixtures
thereof.

7. The nitrogen oxide storage catalyst according to claim 1, wherein the
first washcoat layer further comprises at least one platinum group metal
selected from the group consisting of platinum, palladium, iridium, and
mixtures thereof.

9. The nitrogen oxide storage catalyst according to claim 1, wherein the
nitrogen oxide storage catalyst further comprises a third washcoat layer
which comprises Pd and which is disposed onto at least a portion of the
first washcoat layer, onto which the second washcoat layer is not
disposed.

10. The nitrogen oxide storage catalyst according to claim 9, wherein Pd
comprised in the third washcoat layer is at least partially supported on
metal oxide support particles.

11. The nitrogen oxide storage catalyst according to claim 1, wherein the
substrate comprises a honeycomb substrate comprising a plurality of
longitudinally extending passages formed by longitudinally extending
walls bounding and defining said passages.

12. The nitrogen oxide catalyst according to claim 11, wherein the first
washcoat layer onto 100-x % of the surface of which the second washcoat
layer is disposed is the first washcoat layer of the of the inlet
passages of the honeycomb substrate.

13. The nitrogen oxide catalyst according to claim 11, wherein the
nitrogen oxide storage catalyst comprises a third washcoat layer which
comprises Pd and which is disposed onto at least a portion of the first
washcoat layer, onto which the second washcoat layer is not disposed, and
wherein the first washcoat layer onto the surface of which the third
washcoat layer is disposed is the first washcoat layer of the of the
outlet passages of the honeycomb substrate, said third washcoat layer
being disposed on at least a portion of said first washcoat layer of the
outlet passages of the honeycomb substrate.

14. A treatment system for an automobile exhaust gas stream, comprising:
a combustion engine which operates periodically between lean and rich
conditions; an exhaust gas conduit in communication with the engine; and
a nitrogen oxide storage catalyst according to claim 1 provided within
the exhaust gas conduit.

15. The treatment system according to claim 14, wherein the nitrogen
storage catalyst comprises separate substrates with respect to the
portions or sections of the catalyst which comprise the second washcoat
layer, and those which do not comprise the second washcoat layer.

16. A method for the treatment of automobile engine exhaust gas
comprising: (i) providing a nitrogen oxide storage catalyst according to
claim 1, and (ii) conducting an automobile engine exhaust gas stream over
and/or through the nitrogen oxide storage catalyst.

17. The method according to claim 16, wherein the automobile engine
operates periodically between lean and rich conditions.

18. A method of producing a nitrogen oxide storage catalyst comprising
the steps of: (i) providing a substrate, preferably a honeycomb
substrate; (ii) providing a first washcoat layer on said substrate, said
first washcoat layer comprising metal oxide support particles and at
least one metal compound selected from the group consisting of alkaline
earth metal compounds, alkali metal compounds, rare earth metal
compounds, and mixtures thereof, at least a portion of said at least one
metal compound being supported on the metal oxide support particles;
(iii) drying and/or calcining the coated substrate; (iv) providing a
second washcoat layer comprising Rh on 100-x % of the surface of the
first washcoat layer, wherein x ranges from 20 to 80; (v) drying and/or
calcining the coated substrate.

19. The method of claim 18, further comprising the steps of: (vi)
providing a third washcoat layer onto at least a portion of the surface
of the first washcoat layer, onto which the second washcoat layer has not
been provided; (vii) drying and/or calcining the coated substrate;
wherein alternatively, steps (vi) and (vii) are conducted after step
(iii) and prior to step (iv).

20. The method according to claim 19, wherein step (ii) comprises the
steps of: (a) mixing a solution of the at least one metal compound with
particles of at least one metal compound; (b) drying and/or calcining the
mixture to obtain a composite material; (c) impregnating metal oxide
support particles with a solution comprising Pt; (d) impregnating metal
oxide support particles with a solution comprising Pd; (e) providing and
milling a slurry of the particles obtained in steps (c) and (d); (f)
providing a slurry comprising the milled particles obtained in step (e)
and the composite material obtained in step (b) and milling the resulting
mixture; (g) coating the substrate with the slurry obtained in step (f).

21. The method according to claim 20, wherein step (iv) comprises the
steps of: (aa) impregnating metal oxide support particles with a solution
comprising Rh; (bb) providing and milling a slurry of the impregnated
particles; (cc) coating the substrate with the slurry obtained in step
(bb).

22. The method according to claim 21, wherein step (vi) comprises the
steps of: (dd) impregnating metal oxide support particles with a solution
comprising Pd; (ee) providing and milling a slurry of the impregnated
particles; (ff) coating the substrate with the slurry obtained in step
(ee).

23. The method according to claim 19, wherein the substrate is a
honeycomb substrate, and wherein the method preferably comprises a
further step of: (viii) alternatively closing the inlet our outlet ends
of the honeycomb substrate to form inlet passages having an open inlet
end and a closed outlet end, and outlet passages having a closed inlet
end and an open outlet end.

24. The method according to claim 22, wherein in steps (e), (f), (bb)
and/or (ee) the slurry is milled to a particle size (d90) ranging
from 5 to 20 μm, preferably from 8 to 14 μm, more preferably from 9
to 13 μm, and even more preferably from 10 to 12 μm.

25. The method according to claim 22, wherein in steps (c), (d), (aa)
and/or (dd) the metal oxide support particles are impregnated by an
incipient wetness procedure.

26. The method according to claim 22, wherein in steps (g), (cc) and/or
(ff) the coating is performed by dip coating.

27. The method according to claim 20, wherein the slurry in step (f)
contains substantially no Rh.

28. A nitrogen oxide storage catalyst obtainable by a method according to
claim 18.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C.
§119(e) to U.S. Patent Application No. 61/353,242, filed Jun. 10,
2010, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] Aspect of the present invention relates to an NOx trap with a
reduced Rh loading and to a method for the production of an NOx trap with
a reduced Rh loading, as well as to a treatment system for an automobile
exhaust gas stream and to a method for the treatment of automobile engine
exhaust gas.

BACKGROUND

[0003] The emission of nitrogen oxides ("NOx") from lean-burn engines
must be reduced in order to meet emission regulation standards.
Conventional three-way conversion ("TWC") automotive catalysts are
suitable for abating NOx, carbon monoxide ("CO") and hydrocarbon
("HC") pollutants in the exhaust of engines operated at or near
stoichiometric air/fuel conditions.

[0004] Engines, especially gasoline-fueled engines, are being designed to
operate under lean conditions as a fuel economy measure. Such engines are
referred to as "lean-burn engines". That is, the ratio of air to fuel in
the combustion mixtures supplied to such engines is maintained
considerably above the stoichiometric ratio so that the resulting exhaust
gases are "lean", i.e., the exhaust gases are relatively high in oxygen
content.

[0005] Although lean-burn engines provide enhanced fuel economy, they have
the disadvantage that conventional TWC catalysts are not effective for
reducing NOx emissions from such engines because of excessive oxygen
in the exhaust. Attempts to overcome this problem have included operating
lean-burn engines with brief periods of fuel-rich operation (engines
which operate in this fashion are sometimes referred to as "partial
lean-burn engines"). The exhaust of such engines is treated with a
catalyst/NOx sorbent (nitrogen oxide storage catalyst) which stores
NOx during periods of lean (oxygen-rich) operation, and releases the
stored NOx during the rich (fuel-rich) periods of operation. During
periods of rich (or stoichiometric) operation, the catalyst component of
the catalyst/NOx sorbent promotes the reduction of NOx to
nitrogen by reaction of NOx (including NOx released from the
NOx sorbent) with HC, CO and/or hydrogen present in the exhaust gas.

[0006] WO 2008/067375, for example, discloses a nitrogen oxide storage
catalyst having a washcoat layer provided on a substrate, wherein said
washcoat layer contains Rh and further elements which form a composition
which are active in the abatement of nitrogen oxide by trapping and
conversion thereof.

[0007] There, however, remains a need for improved nitrogen oxide storage
catalysts, in particular with respect to the efficiency in the abatement
of nitrogen oxide at lower temperatures, i.e. during the cold-start
periods of automotive exhaust gas treatment. Furthermore, in view of the
costly platinum group metals contained in nitrogen oxide storage
catalysts, there is a need to reduce the amounts thereof which are
necessary for meeting emission regulation standards.

SUMMARY

[0008] The present invention includes the following embodiments, wherein
these include the specific combinations of embodiments as indicated by
the respective interdependencies defined therein:

Embodiment 1

[0009] A nitrogen oxide storage catalyst comprising:

a substrate; a first washcoat layer disposed on the substrate, the first
washcoat layer comprising metal oxide support particles and a nitrogen
oxide storage material comprising at least one metal compound selected
from the group consisting of alkaline earth metal compounds, alkali metal
compounds, rare earth metal compounds, and mixtures thereof, at least a
portion of said at least one metal compound being supported on the metal
oxide support particles; and a second washcoat layer disposed over the
first washcoat layer, said second washcoat layer comprising Rh, wherein
the first washcoat layer contains substantially no Rh, and wherein the
second washcoat layer is disposed on 100-x % of the surface of the first
washcoat layer, x ranging from 20 to 80, preferably from 25 to 75, more
preferably from 30 to 70, more preferably from 35 to 65, more preferably
from 40 to 60, more preferably from 45 to 55, and even more preferably
from 48 to 52, wherein preferably the first and second washcoat layers
are both disposed on the substrate starting from one end and/or side of
the substrate body, and wherein more preferably the second washcoat layer
is a continuous layer.

Embodiment 2

[0010] The nitrogen oxide storage catalyst according to embodiment 1,

[0011] wherein the second washcoat layer further comprises metal oxide
support particles on which Rh is at least partially supported.

[0013] The nitrogen oxide storage catalyst according to any one of
embodiments 1 to 3, wherein the nitrogen oxide storage material comprises
at least one element selected from the group consisting of Li, Na, K, Rb,
Cs, Mg, Ca, Sr, Ba, Ce, La, Pr, Nd, and mixtures thereof, preferably at
least one element selected from the group consisting of Mg, Ba, Ce, and
mixtures thereof, more preferably Ba and/or Ce.

Embodiment 5

[0014] The nitrogen oxide storage catalyst according to any one of
embodiments 1 to 4, wherein the nitrogen oxide storage material comprises
a cerium compound and a barium compound, preferably ceria and barium
carbonate, preferably in a ceria:barium carbonate weight ratio of from
1:1 to 1:20, more preferably of from 1:2 to 1:15, more preferably of from
1:3 to 1:10, and even more preferably of from 1:3.5 to 1:5.

Embodiment 6

[0015] The nitrogen oxide catalyst according to embodiment 5, wherein
barium carbonate is at least partially supported on particles comprising
ceria, wherein the resulting particles preferably have a particle size
(d90) which ranges from 5 to 20 μm, preferably from 8 to 14 μm,
more preferably from 9 to 13 μm, and even more preferably from 10 to
12 μm.

Embodiment 7

[0016] The nitrogen oxide storage catalyst according to any one of
embodiments 1 to 6, wherein the first washcoat layer further comprises at
least one platinum group metal selected from the group consisting of
platinum, palladium, iridium, and mixtures thereof, wherein the at least
one platinum group metal is preferably at least partially supported on
the metal oxide support particles, more preferably wherein different
types of platinum group metals are supported on separate metal oxide
support particles.

[0018] The nitrogen oxide storage catalyst according to any one of
embodiments 1 to 8, wherein the nitrogen oxide storage catalyst further
comprises a third washcoat layer which comprises Pd and which is disposed
onto at least a portion of the first washcoat layer, onto which the
second washcoat layer is not disposed.

Embodiment 10

[0019] The nitrogen oxide storage catalyst according to embodiment 9,
wherein Pd comprised in the third washcoat layer is at least partially
supported on metal oxide support particles, wherein the metal oxide
support particles preferably comprise at least one metal oxide selected
from the group consisting of alumina, zirconia, zirconia-alumina,
baria-alumina, lanthana-alumina, lanthana-zirconia-alumina, and mixtures
thereof, more preferably, wherein the metal oxide support particles
comprise zirconia-alumina and/or lanthana-alumina, and even more
preferably zirconia-alumina.

Embodiment 11

[0020] The nitrogen oxide storage catalyst according to any one of
embodiments 1 to 10, wherein the substrate comprises a honeycomb
substrate comprising a plurality of longitudinally extending passages
formed by longitudinally extending walls bounding and defining said
passages,

wherein the honeycomb substrate preferably comprises alternating inlet
and outlet passages, said inlet passages having an open inlet end and a
closed outlet end, and said outlet passages having a closed inlet end and
an open outlet end.

Embodiment 12

[0021] The nitrogen oxide catalyst according to embodiment 11, wherein the
first washcoat layer onto 100-x % of the surface of which the second
washcoat layer is disposed is the first washcoat layer of the of the
inlet passages of the honeycomb substrate, wherein the second washcoat
layer is preferably disposed on the first portion of said first washcoat
layer extending from the inlet end of the honeycomb substrate (front
section), wherein more preferably the second washcoat layer is a
continuous layer.

Embodiment 13

[0022] The nitrogen oxide catalyst according to embodiment 11 or 12,
wherein the nitrogen oxide storage catalyst comprises a third washcoat
layer according to embodiment 9 or 10, and wherein the first washcoat
layer onto the surface of which the third washcoat layer is disposed is
the first washcoat layer of the of the outlet passages of the honeycomb
substrate, said third washcoat layer being disposed on at least a portion
of said first washcoat layer of the outlet passages of the honeycomb
substrate.

Embodiment 14

[0023] A treatment system for an automobile exhaust gas stream,
comprising:

[0024] a combustion engine which operates periodically between lean and
rich conditions;

[0025] an exhaust gas conduit in communication with the engine; and

[0026] a nitrogen oxide storage catalyst according to any of embodiments 1
to 13 provided within the exhaust gas conduit.

Embodiment 15

[0027] The treatment system according to embodiment 14, wherein the
nitrogen storage catalyst comprises separate substrates with respect to
the portions or sections of the catalyst which comprise the second
washcoat layer, and those which do not comprise the second washcoat
layer, preferably wherein according to any one of embodiments 9 to 13 the
nitrogen storage catalyst comprises separate substrates with respect to
the portions or sections of the catalyst which comprise the second
washcoat layer, and those which comprise the third washcoat layer.

Embodiment 16

[0028] A method for the treatment of automobile engine exhaust gas
comprising:

[0029] (i) providing a nitrogen oxide storage catalyst according to any
one of embodiments 1 to 13, and

[0034] (ii) providing a first washcoat layer on said substrate, said first
washcoat layer comprising metal oxide support particles and at least one
metal compound selected from the group consisting of alkaline earth metal
compounds, alkali metal compounds, rare earth metal compounds, and
mixtures thereof, at least a portion of said at least one metal compound
being supported on the metal oxide support particles;

[0035] (iii) drying and/or calcining the coated substrate;

[0036] (iv) providing a second washcoat layer comprising Rh on 100-x % of
the surface of the first washcoat layer, wherein x ranges from 20 to 80,
preferably from 25 to 75, more preferably from 30 to 70, more preferably
from 35 to 65, more preferably from 40 to 60, more preferably from 45 to
65, and even more preferably from 48 to 52;

[0037] (v) drying and/or calcining the coated substrate.

Embodiment 19

[0038] The method of embodiment 18, further comprising the steps of:

[0039] (vi) providing a third washcoat layer onto at least a portion of
the surface of the first washcoat layer, onto which the second washcoat
layer has not been provided;

[0040] (vii) drying and/or calcining the coated substrate;

wherein alternatively, steps (vi) and (vii) are conducted after step
(iii) and prior to step (iv).

Embodiment 20

[0041] The method according to embodiment 18 or 19, wherein step (ii)
comprises the steps of: [0042] (a) mixing a solution of the at least
one metal compound with particles of at least one metal compound; [0043]
(b) drying and/or calcining the mixture to obtain a composite material;
[0044] (c) impregnating metal oxide support particles with a solution
comprising Pt; [0045] (d) impregnating metal oxide support particles with
a solution comprising Pd; [0046] (e) providing and milling a slurry of
the particles obtained in steps (c) and (d); [0047] (f) providing a
slurry comprising the milled particles obtained in step (e) and the
composite material obtained in step (b) and milling the resulting
mixture; [0048] (g) coating the substrate with the slurry obtained in
step (f).

Embodiment 21

[0049] The method according to any one of embodiments 18 to 20, wherein
step (iv) comprises the steps of: [0050] (aa) impregnating metal oxide
support particles with a solution comprising Rh; [0051] (bb) providing
and milling a slurry of the impregnated particles; [0052] (cc) coating
the substrate with the slurry obtained in step (bb).

Embodiment 22

[0053] The method according to any one of embodiments 19 to 21, wherein
step (vi) comprises the steps of: [0054] (dd) impregnating metal oxide
support particles with a solution comprising Pd; [0055] (ee) providing
and milling a slurry of the impregnated particles; [0056] (ff) coating
the substrate with the slurry obtained in step (ee).

Embodiment 23

[0057] The method according to any one of embodiments 19 to 22,

[0058] wherein the substrate is a honeycomb substrate, and wherein the
method preferably comprises a further step of:

[0059] (viii) alternatively closing the inlet our outlet ends of the
honeycomb substrate to form inlet passages having an open inlet end and a
closed outlet end, and outlet passages having a closed inlet end and an
open outlet end.

Embodiment 24

[0060] The method according to embodiment 22 or 23, wherein in steps (e),
(f), (bb) and/or (ee) the slurry is milled to a particle size (d90)
ranging from 5 to 20 μm, preferably from 8 to 14 μm, more
preferably from 9 to 13 μm, and even more preferably from 10 to 12
μm.

Embodiment 25

[0061] The method according to any one of embodiments 22 to 24, wherein in
steps (c), (d), (aa) and/or (dd) the metal oxide support particles are
impregnated by an incipient wetness procedure.

Embodiment 26

[0062] The method according to any one of embodiments 22 to 25, wherein in
steps (g), (cc) and/or (ff) the coating is performed by dip coating.

Embodiment 27

[0063] The method according to any one of embodiments 20 to 26, wherein
the slurry in step (f) contains substantially no Rh.

Embodiment 28

[0064] A nitrogen oxide storage catalyst obtainable according to a method
according to any one of embodiments 18 to 27.

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] FIG. 1 is a graph comparing the percent NOx conversion as a
function of the temperature in ° C. for selected examples of the
description, wherein the NOx tailpipe trigger was set to 100 ppm. In FIG.
1, ".diamond-solid." represents the values obtained for Comparative
Example 1, ".box-solid." stands for the values obtained for Example 2,
and ".tangle-solidup." stands for the values obtained for Example 3.

[0066] FIG. 2 is a graph comparing the NOx storage in cycle 10 after 9
lean/rich cycles with a lean NOx trigger set to 100 ppm as a function of
the temperature in ° C. for selected examples of the description,
wherein the NOx tailpipe trigger was set to 100 ppm. In FIG. 2,
".diamond-solid." stands for the values obtained for Comparative Example
1, ".box-solid." stands for the values obtained for Example 2,
".tangle-solidup." stands for the values obtained for Example 3, and "NOx
Storage Cycle 10/g" stands for the NOx Storage Efficiency in g/L.

[0067] FIG. 3 is a graph comparing the percent NOx conversion as a
function of the temperature in ° C. for selected examples of the
description, wherein the NOx tailpipe trigger was set to 100 ppm. In FIG.
3, ".tangle-solidup." stands for the values obtained for Comparative
Example 1, and ".box-solid." stands for the values obtained for Example
4.

[0068] FIG. 4 is a graph comparing the NOx storage in cycle 10 after 9
lean/rich cycles with a lean NOx trigger set to 100 ppm as a function of
the temperature in ° C. for selected examples of the description,
wherein the NOx tailpipe trigger was set to 100 ppm. In FIG. 4,
".tangle-solidup." stands for the values obtained for Comparative Example
1, ".box-solid." stands for the values obtained for Example 4, and "NOx
Storage Cycle 10/g" stands for the NOx Storage Efficiency in g/L.

[0069] FIG. 5 is a graph comparing the percent NOx conversion as a
function of the temperature in ° C. for selected examples of the
description, wherein the NOx tailpipe trigger was set to 40 ppm. In FIG.
5, ".box-solid." stands for the values obtained for Comparative Example
1, and ".diamond-solid." stands for the values obtained for Example 4.

[0070] FIG. 6 is a graph comparing the NOx storage in cycle 10 after 9
lean/rich cycles with a lean NOx trigger set to 40 ppm as a function of
the temperature in ° C. for selected examples of the description,
wherein the NOx tailpipe trigger was set to 40 ppm. In FIG. 6,
".box-solid." stands for the values obtained for Comparative Example 1,
".diamond-solid." stands for the values obtained for Example 4, and "NOx
Storage Cycle 10/g" stands for the NOx Storage in cycle 10 in g/L.

DETAILED DESCRIPTION

[0071] In this respect, it has surprisingly been found that a NOx storage
catalyst design according to embodiments of the present invention which
involves lower loadings of platinum group metals, in particular of Rh,
affords improved performance with respect to the abatement of nitrogen
oxide, especially during the cold-start period of the exhaust treatment
process.

[0072] Thus, an embodiment of the present invention relates to a nitrogen
oxide storage catalyst comprising:

a substrate; a first washcoat layer disposed on the substrate, the first
washcoat layer comprising metal oxide support particles and a nitrogen
oxide storage material comprising at least one metal compound selected
from the group consisting of alkaline earth metal compounds, alkali metal
compounds, rare earth metal compounds, and mixtures thereof, at least a
portion of said at least one metal compound being supported on the metal
oxide support particles; and a second washcoat layer disposed over the
first washcoat layer, said second washcoat layer comprising Rh, wherein
the first washcoat layer contains substantially no Rh, and wherein the
second washcoat layer is disposed on 100-x % of the surface of the first
washcoat layer, x ranging from 20 to 80.

[0073] As the substrate, any material may be used provided that it may
support the washcoat layers of the nitrogen oxide storage catalyst and
that it is resistant to the conditions which reign during the exhaust gas
treatment process. Suitable substrates include any of those materials
typically used for preparing catalysts, and will usually comprise a
ceramic or metal honeycomb structure.

[0074] The substrate according to embodiments of the present invention may
be of any conceivable shape, provided that it allows for the fluid
contact with at least a portion of the washcoat layers present thereon.
Preferably, the substrate is a monolith, wherein more preferably the
monolith is a flow-through monolith. Accordingly, the monolithic
substrate preferably contains fine, parallel gas flow passages extending
from an inlet to an outlet face of the substrate, such that passages are
open to fluid flow. Such substrates are commonly referred to as honeycomb
flow through substrates. The passages, which are essentially straight
paths from their fluid inlet to their fluid outlet, are defined by walls
on which the washcoats are disposed, so that the gases flowing through
the passages contact the catalytic material. The flow passages of the
monolithic substrate are thin-walled channels, which can be of any
suitable cross-sectional shape and size such as trapezoidal, rectangular,
square, sinusoidal, hexagonal, oval, or circular. Such structures may
contain from 60 to 400 or more gas inlet openings (i.e., cells) per
square inch of cross section.

[0075] Thus according to a preferred embodiment of the present invention,
the nitrogen oxide storage catalyst comprises a monolith as the
substrate, preferably a flow-through monolith, and more preferably a
flow-through monolith having a honeycomb-structure.

[0076] The substrate can be made from materials commonly known in the art.
For this purpose, porous materials are preferably used as the substrate
material, in particular ceramic and ceramic-like materials such as
cordierite, α-alumina, an aluminosilicate, cordierite-alumina,
silicon carbide, aluminum titanate, silicon nitride, zirconia, mullite,
zircon, zircon mullite, zircon silicate, sillimanite, a magnesium
silicate, petalite, spodumene, alumina-silica-magnesia and zirconium
silicate, as well as porous refractory metals and oxides thereof.
According to embodiments of the present invention, "refractory metal"
refers to one or more metals selected from the group consisting of Ti,
Zr, Hf, V, Nb, Ta, Cr, Mo, W, and Re. The substrate may also be formed of
ceramic fiber composite materials. According to the present invention,
the substrate is preferably formed from cordierite, silicon carbide,
and/or from aluminum titanate. In general, materials are preferred which
are able to withstand the high temperatures to which a NOx storage
catalyst is exposed to, in particular when used in the treatment of
automotive exhaust gas.

[0077] The substrates useful for the catalysts of embodiments of the
present invention may also be metallic in nature and be composed of one
or more metals or metal alloys. The metallic substrates may be employed
in various shapes such as corrugated sheet or monolithic form. Suitable
metallic supports include the heat resistant metals and metal alloys such
as titanium and stainless steel as well as other alloys in which iron is
a substantial or major component. Such alloys may contain one or more of
nickel, chromium and/or aluminum, and the total amount of these metals
may advantageously comprise at least 15 wt.-% of the alloy, e.g., 10-25
wt.-% of chromium, 3-8 wt.-% of aluminum and up to 20 wt.-% of nickel.
The alloys may also contain small or trace amounts of one or more other
metals such as manganese, copper, vanadium, titanium and the like. The
surface or the metal substrates may be oxidized at high temperatures,
e.g., 1000° C. and higher, to improve the resistance to corrosion
of the alloys by forming an oxide layer on the surfaces the substrates.
Such high temperature-induced oxidation may enhance the subsequent
adherence of the washcoat compositions to the substrate.

[0078] In principle, any metal oxide support particles may be used in the
first washcoat layer, provided that they can withstand the conditions
encountered during the treatment of automotive exhaust gas, in particular
with respect to the temperatures incurred by the NOx storage catalyst.
Preferably, the metal oxide support particles comprise at least one metal
oxide selected from the group consisting of alumina, zirconia,
zirconia-alumina, baria-alumina, lanthana-alumina,
lanthana-zirconia-alumina, and mixtures thereof. More preferably, the
metal oxide particles comprise zirconia-alumina and/or lanthana-alumina,
even more preferably zirconia-alumina.

[0079] According to preferred embodiments of the present invention, the
metal oxide support particles comprised in the first washcoat layer may
be doped with one or more compounds. Thus, the metal oxide support
particles, preferably alumina, comprised in the first washcoat layer are
preferably doped with zirconia. In principle, the metal oxide support
particles may be doped with any possible amount of zirconia, preferably
with 0.5 to 40% zirconia, more preferably with from 1 to 30%, more
preferably with from 5 to 25%, and even more preferably with from 10 to
20%.

[0080] According to embodiments of the present invention, the particle
size (d90) of the metal oxide support particles comprised in the
first washcoat layer preferably ranges from 5 to 20 μm, more
preferably from 8 to 14 μm, more preferably from 9 to 13 μm, and
even more preferably from 10 to 12 μm. Within the meaning of the
present invention a particle size "d90" refers to the equivalent
diameter where 90% of the number of particles per volume have a smaller
diameter.

[0081] Furthermore, according to embodiments of the present invention, the
surface area of the metal oxide support particles may range from 50 to
350 m2/g, wherein preferably, the surface area ranges from 100 to
300 m2/g, more preferably from 130 to 270 m2/g, more preferably
from 150 to 230 m2/g, and even more preferably from 170 to 190
m2/g. Within the meaning of the present invention, the surface area
generally refers to the BET surface area, preferably to the BET surface
area determined according to DIN 66135.

[0082] Regarding the nitrogen oxide storage material comprised in the
first washcoat layer, it is further preferred that said material
comprises at least one element selected from the group consisting of Li,
Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Ce, La, Pr, Nd, and mixtures thereof,
preferably at least one element selected from the group consisting of Mg,
Ba, Ce, and mixtures thereof, more preferably Ba and/or Ce.

[0083] According to a particularly preferred embodiment, the nitrogen
oxide storage material comprises a cerium compound and a barium compound,
preferably ceria and barium carbonate, wherein the ceria:barium carbonate
weight ratio preferably ranges from 1:1 to 1:20, more preferably of from
1:2 to 1:15, more preferably of from 1:3 to 1:10, and even more
preferably of from 1:3.5 to 1:5.

[0084] With respect to said particularly preferred embodiment, barium
carbonate is preferably at least partially supported on particles
comprising ceria, wherein the resulting particles preferably have a
particle size (d90) which ranges from 5 to 20 μm, more preferably
from 8 to 14 μm, more preferably from 9 to 13 μm, and even more
preferably from 10 to 12 μm.

[0085] In general, the nitrogen oxide storage material may have any
possible surface area, wherein according to a preferred embodiment, the
surface area of the nitrogen oxide storage material ranges from 25 to 100
m2/g, more preferably from 30 to 80 m2/g, more preferably from
40 to 60 m2/g, and even more preferably from 45 to 55 m2/g.

[0086] According to embodiments of the present invention, it is further
preferred that the first washcoat layer comprises at least one platinum
group metal selected from the group consisting of platinum, palladium,
iridium, and mixtures thereof. Preferably, the at least one platinum
group metal is at least partially supported on the metal oxide support
particles, wherein more preferably, different types of platinum group
metals are supported on separate metal oxide support particles. In
particular, it is preferred that the first washcoat layer comprises Pd
and/or Pt, preferably Pd and Pt.

[0087] In preferred embodiments of the present invention wherein the
nitrogen oxide storage catalyst comprises Pd, it is further preferred
that the total loading of Pd in the catalyst ranges from 5 to 25
g/ft3. More preferably, the total loading of Pd in the nitrogen
oxide storage catalyst ranges from 8 to 20 g/ft3, more preferably
from 10 to 17 g/ft3, more preferably from 11 to 16 g/ft3, more
preferably from 12 to 15 g/ft3, and even more preferably from 13 to
14 g/ft3

[0088] Furthermore, in preferred embodiments of the present invention,
wherein the nitrogen oxide storage catalyst comprises Pt, it is further
preferred that the total loading of Pt in the catalyst is ranges from 5
to 200 g/ft3. More preferably, the total loading of Pt in the
nitrogen oxide storage catalyst ranges from 20 to 150 g/ft3, more
preferably from 40 to 120 g/ft3, more preferably from 50 to 100
g/ft3, more preferably from 60 to 80 g/ft3, and even more
preferably from 65 to 75 g/ft3.

[0089] Concerning the total loading of the first washcoat layer according
to embodiments of the present invention, any possible loading may be
chosen in principle. In general, the loading of the first washcoat layer
in the nitrogen oxide storage catalyst may range from 0.5 to 20
g/in3, wherein the loading preferably ranges from 1 to 15
g/in3, more preferably from 3 to 10 g/in3, more preferably from
4 to 8 g/in3, more preferably from 5 to 7 g/in3, and even more
preferably from 5.5 to 6.0 g/in3.

[0090] According to embodiments of the present invention, the first
washcoat layer of the nitrogen oxide storage catalyst comprises
substantially no Rh. Within the meaning of the present invention, a
material is preferably defined as not comprising a substantial amount of
a specific element when it contains 0.001 wt.-% or less of said element,
preferably 0.0005 wt.-% or less, more preferably 0.00001 wt.-% or less,
more preferably 0.000005 wt.-% or less, and even more preferably 0.000001
wt.-% or less thereof.

[0091] Regarding the second washcoat layer, said layer is disposed on
100-x % of the surface of the first washcoat layer, wherein x ranges from
20 to 80, x preferably ranging from 25 to 75, more preferably from 30 to
70, more preferably from 35 to 65, more preferably from 40 to 60, more
preferably from 45 to 55, and even more preferably from 48 to 52.

[0092] According to the nitrogen oxide storage catalyst of the present
invention, the washcoat layers are provided on the substrate in any
conceivable fashion, wherein the first and second washcoat layers are
preferably both disposed on the substrate starting from one end and/or
side of the substrate body. According to a particularly preferred
embodiment, said layers are provided on the substrate as continuous
layers, wherein a continuous layer according to the present invention
preferably refers to a layer which is uninterrupted along its entire
length.

[0093] In general, the loading of Rh in the second washcoat layer may have
any possible value, wherein the loading may range from 0.5 to 10
g/ft3. According to a preferred embodiment, the loading of Rh in the
second washcoat layer ranges from 1 to 8 g/ft3, more preferably from
2 to 6 g/ft3, more preferably from 2.5 to 5.5 g/ft3, more
preferably from 3 to 5 g/ft3, and even more preferably from 3.5 to
4.5 g/ft3.

[0094] Regarding the total loading of Rh in the nitrogen oxide storage
catalyst according to embodiments of the present invention, said total
loading may range from 0.25 to 5 g/ft3. According to a preferred
embodiment, the total loading of Rh ranges from 0.5 to 5 g/ft3, more
preferably from 1 to 3 g/ft3, more preferably from 1.25 to 2.75
g/ft3, more preferably from 1.5 to 2.5 g/ft3, and even more
preferably from 1.75 to 2.25 g/ft3.

[0095] Concerning the total loading of the second washcoat layer according
to the present invention, any possible loading may be chosen in
principle. In general, the loading of the second washcoat layer in the
nitrogen oxide storage catalyst may range from 0.05 to 5 g/in3,
preferably from 0.1 to 2 g/in3, more preferably from 0.2 to 1.5
g/in3, more preferably from 0.3 to 1 g/in3, more preferably
from 0.4 to 0.6 g/in3, and even more preferably from 0.45 to 0.55
g/in3. According to embodiments of the present invention, it is
particularly preferred that the loading of the second washcoat layer in
the portion of the catalyst containing the second washcoat layer is less
than the loading of the first washcoat layer onto which it is disposed.

[0096] According to embodiments of the present invention, it is further
preferred that the second washcoat layer comprises metal oxide support
particles on which Rh is at least partially supported.

[0097] In principle, any metal oxide particles may be used in the second
washcoat layer, wherein metal oxide support particles are preferred which
comprise at least one metal oxide selected from the group consisting of
alumina, zirconia, zirconia-alumina, baria-alumina, lanthana-alumina,
lanthana-zirconia-alumina, and mixtures thereof, preferably wherein the
metal oxide support particles comprise zirconia-alumina and/or
lanthana-alumina, more preferably zirconia-alumina.

[0098] As for the metal oxide particles of the first washcoat layer, the
metal oxide support particles comprised in the second washcoat layer may
be doped with one or more compounds. Thus, the metal oxide support
particles, preferably alumina, comprised in the second washcoat layer are
preferably doped with zirconia. In principle the metal oxide support
particles of the second washcoat layer may be doped with any possible
amount of zirconia, wherein the metal oxide support particles are
preferably doped with 0.5 to 40% zirconia, more preferably with from 1 to
30%, more preferably with from 5 to 25%, and even more preferably with
from 10 to 20% zirconia.

[0099] According to particularly preferred embodiments of the present
invention, the first and second washcoat layer of the nitrogen oxide
storage catalyst comprise alumina doped with zirconia, wherein more
preferably alumina comprised in the first washcoat layer is alumina doped
with from 1 to 30% zirconia, more preferably from 5 to 20%, more
preferably from 7 to 15%, more preferably from 8 to 12%, and even more
preferably from zirconia 9 to 11%, and alumina comprised in the second
washcoat layer is alumina doped with from 1 to 50% zirconia, more
preferably from 5 to 40%, more preferably from 10 to 30%, more preferably
from 15 to 25%, more preferably from 18 to 22%, and even more preferably
from zirconia 19 to 21%.

[0100] According to the present invention, it is further preferred that
the particle size (d90) of the metal oxide support particles
comprised in the second washcoat layer ranges from 5 to 20 μm,
preferably from 8 to 14 μm, more preferably from 9 to 13 μm, and
even more preferably from 10 to 12 μm.

[0101] According to the present invention, embodiments are further
preferred wherein a third washcoat layer is disposed onto at least a
portion of the first washcoat layer onto which the second washcoat layer
is not disposed, wherein said third washcoat layer comprises Pd.

[0102] In general, the loading of Pd in the third washcoat layer may have
any possible value, wherein the loading may range from 5 to 30
g/ft3. According to a preferred embodiment, the loading of Pd in the
third washcoat layer ranges from 10 to 25 g/ft3, more preferably
from 13 to 21 g/ft3, more preferably from 15 to 19 g/ft3, more
preferably from 16 to 18 g/ft3, and even more preferably from 16.5
to 17.5 g/ft3.

[0103] Concerning the total loading of the third washcoat layer according
to the present invention, any possible loading may be chosen in
principle. In general, the loading of the third washcoat layer in the
nitrogen oxide storage catalyst may range from 0.05 to 5 g/in3,
preferably from 0.1 to 2 g/in3, more preferably from 0.2 to 1.5
g/in3, more preferably from 0.3 to 1 g/in3, more preferably
from 0.4 to 0.6 g/in3, and even more preferably from 0.45 to 0.55
g/in3. Furthermore, it is preferred that the loading of the third
washcoat layer in the portion of the catalyst containing the third
washcoat layer is less than the loading of the first washcoat layer onto
which it is disposed.

[0104] According to preferred embodiments of the present invention wherein
the nitrogen oxide storage catalyst comprises a third washcoat layer, it
is preferred that the third washcoat layer comprises substantially no Rh.

[0105] In a preferred embodiment of the present invention, Pd comprised in
the third washcoat layer is at least partially supported on metal oxide
particles.

[0106] In principle, any metal oxide support particles may be used in the
third washcoat layer, wherein metal oxide support particles are preferred
which comprise at least one metal oxide selected from the group
consisting of alumina, zirconia, zirconia-alumina, baria-alumina,
lanthana-alumina, lanthana-zirconia-alumina, and mixtures thereof, more
preferably, wherein the metal oxide particles comprise zirconia-alumina
and/or lanthana-alumina, even more preferably zirconia-alumina.

[0107] As for the metal oxide particles of the first and second washcoat
layers, the metal oxide support particles comprised in the third washcoat
layer may be doped with one or more compounds. Thus, the metal oxide
support particles, preferably alumina, comprised in the third washcoat
layer are preferably doped with zirconia. In principle the metal oxide
support particles of the third washcoat layer may be doped with any
possible amount of zirconia, wherein the metal oxide support particles
are preferably doped with 1 to 50% zirconia, more preferably from 5 to
40%, more preferably from 10 to 30%, more preferably from 15 to 25%, more
preferably from 18 to 22%, and even more preferably from zirconia 19 to
21%.

[0108] Concerning the preferred substrate of the nitrogen oxide storage
catalyst of the present invention which is a honeycomb substrate
comprising a plurality of longitudinally extending passages formed by
longitudinally extending walls bounding and defining said passages, it is
further preferred that said honeycomb substrate comprises alternating
inlet and outlet passages, said inlet passages having an open inlet end
and a closed outlet end, and said outlet passages having a closed inlet
end and an open outlet end, thus forming a wall-flow substrate. With
respect to the preferred embodiments of the present invention which
comprise such a wall-flow substrate, it will be understood that the
loading of the washcoat layers thereon will depend on substrate
properties such as porosity and wall thickness.

[0109] According to said preferred embodiments, it is further preferred
that the first washcoat layer onto 100-x % of the surface of which the
second washcoat layer is disposed is the first washcoat layer disposed on
the walls of the inlet passages of the honeycomb substrate, wherein the
second washcoat layer is preferably disposed on the first portion of said
first washcoat layer extending from the inlet end of the honeycomb
substrate (front section). According to the present invention it is
preferred that said second washcoat layer is provided on the substrate as
a continuous layer.

[0110] Regarding preferred embodiments of the nitrogen oxide storage
catalyst which comprise a third washcoat layer, it is further preferred
that the first washcoat layer onto which said third washcoat layer is
disposed is the first washcoat layer disposed on the outlet passages of
the honeycomb substrate, said third washcoat layer being disposed on at
least a portion of said first washcoat layer of the outlet passages of
the honeycomb substrate

[0111] According to a particularly preferred embodiment comprising a
wall-flow substrate, the third washcoat layer is disposed on 100-x % of
the surface of the first washcoat layer, x ranging from 20 to 80,
preferably from 25 to 75, more preferably from 30 to 70, more preferably
from 35 to 65, more preferably from 40 to 60, more preferably from 45 to
65, and even more preferably from 48 to 52. Furthermore, said third
washcoat layer is preferably disposed on a portion of said first washcoat
layer extending from the outlet end of the honeycomb substrate (rear
section).

[0112] In addition to the above-mentioned nitrogen oxide storage catalyst,
an embodiment of the present invention is also directed to a treatment
system for an automobile exhaust gas stream. In particular, the treatment
system of the present invention comprises

[0113] a combustion engine which operates periodically between lean and
rich conditions;

[0114] an exhaust gas conduit in communication with the engine; and

[0115] a nitrogen oxide storage catalyst according to the present
invention provided within the exhaust gas conduit

[0116] In principle, any conceivable combustion engine may be used in the
treatment system of the present invention, wherein preferably a gasoline
engine is used, and more preferably a direct injection gasoline engine.

[0117] In a gasoline vehicle application, a catalytic device employing a
three-way conversion ("TWC") catalyst may be used in conjunction with the
nitrogen oxide storage catalyst of the present invention. Such a device
may be located within the exhaust gas conduit of the internal combustion
engine and will be disposed upstream and/or downstream of the nitrogen
oxide storage catalyst. The TWC catalyst typically includes platinum,
palladium and rhodium catalytic components dispersed on a high surface
area refractory support and may also contain one or more base metal oxide
catalytic components such as oxides of iron, manganese or nickel. Such
catalysts can be stabilized against thermal degradation by expedients
such as impregnating an activated alumina support with one or more rare
earth metal oxides such as ceria. Such stabilized catalysts can sustain
very high operating temperatures. For example, if a fuel cut technique is
utilized, temperatures as high as 1050° C. may be sustained in the
catalytic device.

[0118] If the catalytic device is employed and is located upstream of the
nitrogen oxide storage catalyst of the invention, the catalytic device
may be mounted close to the exhaust manifold of the engine. In such an
arrangement, the TWC catalyst may warm up quickly and provide for
efficient cold start emission control. Once the engine is warmed up, the
TWC catalyst will remove HC, CO and NOx from the exhaust gas stream
during stoichiometric or rich operation and HC and CO during lean
operation. In one embodiment, the nitrogen oxide storage catalyst is
positioned downstream of the catalytic device where the exhaust gas
temperature enables maximum NOx trap efficiency. During periods of
lean engine operation, when NOx passes through the TWC catalyst,
NOx is stored on the nitrogen oxide storage catalyst. The nitrogen
oxide is then periodically desorbed and the NOx is reduced to
nitrogen under periods of stoichiometric or rich engine operation. If
desired, a catalytic device containing a TWC catalyst may be employed
downstream of the nitrogen oxide storage catalyst of the invention. Such
a catalytic device may serve to remove further amounts of HC and CO from
the exhaust gas stream and, in particular, will provide for efficient
reduction of the NOx to nitrogen under periods of stoichiometric or
rich engine operation.

[0119] In a diesel vehicle application, the nitrogen oxide storage
catalyst according to the present invention may be used in conjunction
with a diesel oxidation catalyst (DOC), and a catalyzed soot filter
(CSF), where the DOC and CSF are placed either before or after the
nitrogen oxide storage catalyst.

[0120] Furthermore, embodiments of the treatment system are preferred
which, in addition to or alternatively to the three way catalyst,
comprise a selective reduction catalyst provided upstream or downstream
from the nitrogen oxide storage catalyst, wherein embodiments are
preferred with the selective reduction catalyst provided downstream from
the nitrogen oxide storage catalyst. According to a particularly
preferred embodiment, the treatment system of the present invention
comprises both a three way catalyst and a selective reduction catalyst
located downstream from the nitrogen oxide storage catalyst, wherein
preferably, both the three way catalyst and the selective reduction
catalysts are located on the same substrate.

[0121] According to certain embodiments of the present invention, the
nitrogen storage catalyst may be comprised on separate substrates with
respect to the portions or sections of the catalyst which comprise the
second washcoat layer, and those which do not comprise the second
washcoat layer. With respect to embodiments of the present invention
which comprise a third washcoat layer, it is preferred that the nitrogen
storage catalyst comprises separate substrates with respect to the
portions or sections of the catalyst which comprise the second washcoat
layer, and those which comprise the third washcoat layer.

[0122] In addition to these embodiments, the present invention also
relates to a method for the treatment of automobile engine exhaust gas
using the nitrogen oxide storage catalyst of the present invention. More
specifically, the method of the present invention includes conducting an
automobile engine exhaust gas over and/or through the nitrogen oxide
storage catalyst, wherein the automobile engine exhaust gas is preferably
conducted through the nitrogen oxide storage catalyst.

[0123] Thus, the present invention also concerns a method for the
treatment of automobile engine exhaust gas comprising:

[0124] (i) providing a nitrogen oxide storage catalyst according to the
present invention, and

[0126] In the method of the present invention, it is preferred that the
automobile engine exhaust gas is from a gasoline engine, more preferably
from a direct injection gasoline engine.

[0127] In use, the exhaust gas stream which is contacted with the nitrogen
oxide storage catalyst of the present invention is alternately adjusted
between lean and stoichiometric/rich operating conditions so as to
provide alternating lean operating periods and stoichiometric/rich
operating periods. It will be understood that the exhaust gas stream
being treated may be selectively rendered lean or stoichiometric/rich
either by adjusting the air-to-fuel ratio fed to the engine generating
the exhaust or by periodically injecting a reductant into the gas stream
upstream of the catalytic trap. For example, the composition of
embodiments of the present invention is well suited to treat the exhaust
of engines, including diesel engines, which continuously run lean. In
such case, in order to establish a stoichiometric/rich operating period,
a suitable reductant, such as fuel, may be periodically sprayed into the
exhaust immediately upstream of the nitrogen oxide storage catalyst of
the present invention to provide at least local (at the nitrogen oxide
storage catalyst) stoichiometric/rich conditions at selected intervals.
Partial lean-burn engines, such as partial lean-burn gasoline engines,
are designed with controls which cause them to operate lean with brief,
intermittent rich or stoichiometric conditions.

[0128] Thus, according to the method of embodiments of the present
invention, it is preferred that the automobile engine is operated
periodically between lean and rich conditions.

[0129] In addition to the nitrogen oxide storage catalyst described in the
foregoing, an aspect of the present invention also relates to a method
for its production. In particular, an embodiment of the present invention
further relates to a method of producing a nitrogen oxide storage
catalyst comprising the steps of:

[0130] (i) providing a substrate which is preferably a honeycomb
substrate;

[0131] (ii) providing a first washcoat layer on said substrate, said first
washcoat layer comprising metal oxide support particles and at least one
metal compound selected from the group consisting of alkaline earth metal
compounds, alkali metal compounds, rare earth metal compounds, and
mixtures thereof, at least a portion of said at least one metal compound
being supported on the metal oxide support particles;

[0132] (iii) drying and/or calcining the coated substrate;

[0133] (iv) providing a second washcoat layer comprising Rh on 100-x % of
the surface of the first washcoat layer, wherein x ranges from 20 to 80,
preferably from 25 to 75, more preferably from 30 to 70, more preferably
from 35 to 65, more preferably from 40 to 60, more preferably from 45 to
65, and even more preferably from 48 to 52;

[0134] (v) drying and/or calcining the coated substrate.

[0135] According to the production method of an embodiment of the present
invention, the washcoat layers may be provided on the substrate by any
means commonly used in the art, wherein preferably the washcoat is
applied to the substrate by a dip coating procedure. In general, the
preferred dip coating procedure may be conducted once to apply the
washcoat layer in question, and may be repeated as many times as
necessary for achieving the desired loading of said washcoat layer.

[0136] Concerning the drying procedure used in the production method of an
embodiment of the present invention, the temperature and duration of said
procedure are generally chosen such that the resulting dried product is
essentially devoid of any solvent employed in the coating procedure.

[0137] With respect to the calcining procedure used in the production
method of an embodiment of the present invention, the temperature and
duration are generally chosen such that a product is obtained which
displays the chemical and physical transformations typical to the
calcination process. Preferably, the temperature at which the calcination
procedure is conducted is comprised in the range of from 450 to
600° C., more preferably of from 500 to 580° C., and even
more preferably of from 540 to 560° C. The calcination procedure
may be conducted under any suitable atmosphere, wherein the calcination
is generally conducted under air.

[0138] According to the inventive production method, it is preferred that
the metal oxide support particles comprise at least one metal oxide
selected from the group consisting of alumina, zirconia,
zirconia-alumina, baria-alumina, lanthana-alumina,
lanthana-zirconia-alumina, and mixtures thereof, wherein more preferably
the metal oxide support particles comprise zirconia-alumina and/or
lanthana-alumina, even more preferably zirconia-alumina.

[0139] According to a preferred embodiment, step (ii) of the inventive
production method comprises the steps of: [0140] (a) mixing a solution
of at least one metal compound with particles of at least one metal
compound; [0141] (b) drying and/or calcining the mixture to obtain a
composite material; [0142] (c) impregnating metal oxide support particles
with a solution comprising Pt; [0143] (d) impregnating metal oxide
support particles with a solution comprising Pd; [0144] (e) providing and
milling a slurry of the particles obtained in steps (c) and (d); [0145]
(f) providing a slurry comprising the milled particles obtained in step
(e) and the composite material obtained in step (b) and milling the
resulting mixture; [0146] (g) coating the substrate with the slurry
obtained in step (f).

[0147] According to the present invention, it is preferred that the slurry
in step (f) contains substantially no Rh.

[0148] According to the inventive production process, it is further
preferred that in step (a) the at least one metal compound in solution
and the at least one metal compound present as particles do not contain
the same metals, wherein more preferably the at least one metal compound
in solution comprises Ba, and the at least one metal compound in particle
form comprises Ce, more preferably, wherein said particles comprise
ceria.

[0149] Regarding the calcining procedure used in step (b), the temperature
employed therein is preferably comprised in the range of from 500 to
800° C., more preferably of from 600 to 750° C., and even
more preferably of from 680 to 720° C.

[0150] With respect to the impregnation of metal oxide support particles
with platinum group metal containing solutions in steps (c) and/or (d) of
the preferred method, said procedure may be conducted in any known
manner, provided that the platinum group metal respectively comprised
therein may be effectively supported onto the metal oxide support
particles. Preferably, this is achieved by an incipient wetness
procedure.

[0151] According to a further embodiment of the present production method,
a solution comprising the platinum group metals according to steps (c)
and (d) may be directly added together with the metal oxide particles and
the composite material obtained in step (b) to form a slurry which is
then milled according to step (e). Thus, according to an alternative
embodiment of the present invention which is preferred, step (ii)
comprises the steps of:

[0152] (a) mixing a solution of at least one metal compound with particles
of at least one metal compound;

[0154] (f') providing a slurry comprising a solution comprising Pt and Pd,
metal oxide support particles, and the composite material obtained in
step (b) and milling the resulting mixture;

[0155] (g) coating the substrate with the slurry obtained in step (f').

[0156] Regarding the milling procedure used in preferred embodiments of
the inventive production process, said milling may be conducted in any
known milling apparatus suited for the milling of particles as used in
the present invention, wherein a milling apparatus is preferably employed
which may grind the particles down to an average particle size (d90)
ranging from 5 to 20 μm, preferably from 8 to 14 μm, more
preferably from 9 to 13 μm, and even more preferably from 10 to 12
μm.

[0157] According to preferred embodiments of the inventive method, a
slurry is provided for the milling and the application of the components
comprised in the respective washcoat layers onto the substrate. In
general, the slurry employed in the preferred embodiments of the
inventive method may be provided according to any method known in the art
using any suitable solvent, wherein aqueous solvents and in particular
water, preferably as distilled water, are preferably used.

[0158] According to the inventive production method, it is further
preferred that step (iv) comprises the steps of: [0159] (aa)
impregnating metal oxide support particles with a solution comprising Rh;
[0160] (bb) providing and milling a slurry of the impregnated particles;
[0161] (cc) coating the substrate with the slurry obtained in step (bb).

[0162] Concerning steps (aa), (bb), and (cc) of said preferred embodiment,
it is further preferred that these respectively be conducted in the same
fashion as steps (c) or (d), i.e. with respect to step (aa), (e), i.e.
with respect to step (bb), and (g), i.e. with respect to step (cc).

[0163] According to a preferred embodiment, the inventive further
comprises the steps of:

[0164] (vi) providing a third washcoat layer onto at least a portion of
the surface of the first washcoat layer, onto which the second washcoat
layer has not been provided; (vii)

[0165] drying and/or calcining the coated substrate.

[0166] Furthermore, according to said preferred embodiment, the steps (vi)
and (vii) may either be conducted after step (v) or, alternatively, after
step (iii) and prior to step (iv) of the inventive production method.

[0169] (ee) providing and milling a slurry of the impregnated particles;

[0170] (ff) coating the substrate with the slurry obtained in step (ee).

[0171] As for steps (aa), (bb) and (cc), it is further preferred that
steps (dd), (ee), and (ff) respectively be conducted in the same fashion
as steps (c) or (d), with respect to step (dd), (e), with respect to step
(ee), and (g) with respect to step (ff).

[0172] According to the preferred embodiments of the inventive production
method, it is further preferred that in steps (aa) and/or (dd) the metal
oxide support particles are impregnated by an incipient wetness
procedure. Furthermore, it is preferred that in steps (g), (cc) and/or
(ff) the coating is achieved by dip coating.

[0173] According to embodiments of the inventive production method which
are further preferred, the substrate is a honeycomb substrate, and the
method comprises a further step of:

[0174] (viii) alternatively closing the inlet our outlet ends of the
honeycomb substrate to form inlet passages having an open inlet end and a
closed outlet end, and outlet passages having a closed inlet end and an
open outlet end.

[0175] According to the inventive method, it is particularly preferred
that in steps (e), (f), (f'), (bb) and/or (ee) the slurry is milled to a
particle size (d90) ranging from 5 to 20 μm, preferably from 8 to
14 μm, more preferably from 9 to 13 μm, and even more preferably
from 10 to 12 μm.

[0176] Even when not explicitly mentioned, the inventive method for the
production of a nitrogen oxide storage catalyst is to be understood as
comprising all of the features obviously necessary for obtaining the
nitrogen oxide storage catalyst of the present invention, in particular
with respect to the type and amount as well as with respect to the
chemical and physical properties of the individual components comprised
therein.

[0177] In addition to this, the present invention relates to the products
of the inventive production method per se, in particular with respect to
the chemical and physical properties of a nitrogen oxide storage catalyst
which is obtainable according to said method. Therefore, the present
invention also relates to a nitrogen oxide catalyst obtainable according
to the inventive method of producing a nitrogen oxide storage catalyst.

EXAMPLES

Comparative Example 1

Preparation of the Nox-Trap Layer

[0178] BaCO3 and CeO2 were intimately mixed and finely dispersed in a
weight ratio of 1:4. To this purpose, cerium oxide having a BET surface
area of from 150 m2/g was mixed with a solution of barium acetate such
that the BaCO3/CeO2 composite had a BaCO3 content of from 25 wt.-%. After
mixing, the suspension of soluble barium acetate and CeO2 was then dried
at a temperature of from 120° C. to obtain a solid mixture of
barium acetate and ceria.

[0179] After drying, the mixture was then heated at 700° C. for 2
hours to form particles of ceria having barium carbonate supported on the
ceria particles. The resulting BaCO3 had an average crystallite size of
from about 25 nm and the ceria had an average crystallite size of 10 nm.
The BaCO3/CeO2 composite formed particles with an average size of from
about 10 microns. The BET surface area of the particulate mixture is 50
m2/g.

Preparation of Catalytic Component

[0180] To provide a fully formulated NOx storage catalyst or catalytic
trap as described above, in addition to the manufacture of barium
carbonate supported on ceria, a precious metal is supported on a
refractory oxide according to the following procedure.

[0181] Pt and Rh are impregnated onto Al2O3 by an incipient wetness
procedure to yield 1.8 wt.-% Pt and 0.3 wt.-% Rh. Pd is impregnated
separately onto alumina to a Pd loading of 1.4 wt.-%. In both cases, the
alumina had a BET surface area of 200 m2/g and contained 10 wt.-%
zirconia.

[0182] A mixture of 1.65 g/in3 of the Pt/Rh alumina and 0.4 g/in3 Pd on
alumina was prepared. A solution of zirconium acetate with a content of
0.2 g/in3 was added, giving a slurry with a solid content of 45%. This
slurry was milled with a ball mill until a particle size of 12 micron
(d90) was obtained. Magnesium acetate was added to the slurry and stirred
to dissolve, yielding 0.6 g/in3 magnesium oxide. To this mixture, 3.4
g/in3 of the BaCO3/CeO2 composite particles is added and the slurry is
milled at pH 6.5-7 until a particle size of 11 micron (d90) is obtained.

Coating of a Substrate

[0183] A ceramic honeycomb substrate was coated with the slurry in a dip
coating manner and then dried in a dryer and subsequently calcined in a
furnace under air at 550° C. The coating procedure was then
repeated until a loading of 6.3 g/in3 is achieved.

[0184] The final nitrogen oxide storage catalyst displays a platinum
loading of 72 g/ft3, a rhodium loading of 3.6 g/ft3, and a palladium
loading of 14.4 g/ft3.

Example 2

[0185] Samples according to the present example were prepared with the
addition of a second layer, as described below.

[0186] Pt is impregnated onto Al2O3 by an incipient wetness procedure to
yield 1.8 wt.-% Pt. Pd is impregnated separately onto alumina to a Pd
loading of 0.2 wt.-%. In both cases, the alumina had a BET surface area
of 200 m2/g and contained 10 wt.-% zirconia.

[0187] A mixture of 1.5 g/in3 of the Pt alumina and 0.4 g/in3 Pd on
alumina was prepared. A solution of zirconium acetate with a content of
0.15 g/in3 was added, giving a slurry with a solid content of 42%. This
slurry was milled with a ball mill until a particle size of 12 micron
(d90) was obtained. Magnesium acetate was added to the slurry and stirred
to dissolve, yielding 0.4 g/in3 magnesium oxide. To this mixture, 3.3
g/in3 of BaCO3/CeO2 composite particles obtained according to the
procedure of Comparative Example 1 was added and the slurry was milled at
pH 5-6 until a particle size of 11 micron (d90) is obtained.

Coating of a Substrate

[0188] A ceramic honeycomb substrate was coated with the slurry in a dip
coating manner and then dried in a dryer and subsequently calcined in a
furnace under air at 550° C.

[0189] The coating procedure is then repeated until a loading of from 5.8
g/in3 is achieved.

Preparation of a Second Layer

[0190] A precious metal is impregnated onto alumina with a BET surface
area of 180 m2/g. The alumina is doped with 20 wt.-% zirconia. Rhodium
nitrate is impregnated onto the alumina to yield 0.5 wt.-% Rh. After
impregnation, the alumina slurry is diluted to 35% solids with water. The
pH is adjusted to 3.5 to 4 using tartaric acid. The slurry is then milled
to about 12 micron (d90) with a continuous mill. Subsequently, the pH is
adjusted to 6.5 using MEA.

[0191] To create the second, or subsequent layer, the coated substrate is
coated again with the slurry in a dip coating manner and then dried in a
dryer. The substrate is then calcined in a furnace under air at
550° C. The coating procedure yields the additional coat with a
loading of 0.5 g/in3.

[0192] The overall coating weight of the first and second washcoat layers
in the final nitrogen oxide storage catalyst containing said layers is
6.3 g/in3.

[0193] The final nitrogen oxide storage catalyst displays a platinum
loading of 70 g/ft3, a rhodium loading of 4 g/ft3, and a palladium
loading of 10 g/ft3.

Example 3

[0194] Samples according to the present example were prepared with the
addition of a second layer (the "third washcoat layer" according to the
invention), as described below.

[0195] Pt is impregnated onto Al2O3 by an incipient wetness procedure to
yield 1.8 wt.-% Pt. Pd is impregnated separately onto alumina to a Pd
loading of 0.2 wt.-%. In both cases, the alumina had a BET surface area
of 200 m2/g and contained 10 wt.-% zirconia.

[0196] A mixture of 1.5 g/in3 of the Pt alumina and 0.4 g/in3 Pd on
alumina was prepared. A solution of zirconium acetate with a content of
0.15 g/in3 was added, giving a slurry with a solid content of 42%. This
slurry was milled with a ball mill until a particle size of 12 micron
(d90) was obtained. Magnesium acetate was added to the slurry and stirred
to dissolve, yielding 0.4 g/in3 magnesium oxide. To this mixture, 3.3
g/in3 of BaCO3/CeO2 composite particles obtained according to the
procedure of Comparative Example 1 was added and the slurry was milled at
pH 5-6 until a particle size of 11 micron (d90) is obtained.

Coating of a Substrate

[0197] A ceramic honeycomb substrate was coated with the slurry in a dip
coating manner and then dried in a dryer and subsequently calcined in a
furnace under air at 550° C.

[0198] The coating procedure is then repeated until a loading of from 5.8
g/in3 is achieved.

[0199] Preparation of a Second Layer (the "Third Washcoat Layer" According
to the Invention)

[0200] A precious metal is impregnated onto alumina with a BET surface
area of 180 m2/g. The alumina is doped with 20 wt.-% of zirconia.
Palladium nitrate is impregnated onto the alumina to yield 0.5 wt.-% Pd.
After impregnation, the alumina slurry is diluted to 35% solids with
water. The pH is adjusted to 3.5 to 4 using tartaric acid. The slurry is
then milled to about 12 micron (d90) with a continuous mill.
Subsequently, the pH is adjusted to 6.5 using MEA.

[0201] To create the second, or subsequent layer, the coated substrate is
coated again with the slurry in a dip coating manner and then dried in a
dryer. The substrate is then calcined in a furnace under air at
550° C. The coating procedure yields the additional coat with a
loading of 0.5 g/in3.

[0202] The overall coating weight of the first and second layer in the
final nitrogen oxide storage catalyst is 6.3 g/in3.

[0203] The final nitrogen oxide storage catalyst displays a platinum
loading of 70 g/ft3 and a palladium loading of 17 g/ft3.

Example 4

[0204] Examples 2 and 3 were respectively repeated, wherein the second
layer coating according to Example 2 was only provided on 50% of the
first layer coating, such that 50% of the front section of the honeycomb
substrate relative to the longitudinally extending passages was coated
therewith, and the second layer coating (the "third washcoat layer"
according to the invention) according to Example 3 was provided on the
remaining 50% of the first layer coating which had not been coated with a
second layer coating according to Example 2, such that 50% of the rear
section of the honeycomb substrate was coated therewith.

[0205] The final nitrogen oxide storage catalyst displays a rhodium
loading of 4 g/ft3 in the front section of the catalyst containing the
second layer coating according to Example 2, of 0 g/ft3 in the section of
the catalyst containing the second washcoat layer (the "third washcoat
layer" according to the invention) according to Example 3, and a total
loading of rhodium in the catalyst of 2 g/ft3. The palladium loading is
of 10 g/ft3 in the front section of the catalyst containing the second
layer coating according to Example 2, of 17 g/ft3 in the section of the
catalyst containing the second washcoat layer (the "third washcoat layer"
according to the invention) according to Example 3, and a total loading
of palladium in the catalyst of 13.5 g/ft3.

Example 5

[0206] Example 2 and Comparative Example 1 (modification: no impregnation
with Rh) were respectively repeated, wherein the layer coatings according
to Example 2 were only provided on 50% of the honeycomb substrate, such
that 50% of the front section of the honeycomb substrate relative to the
longitudinally extending passages was coated according to Example 2, and
the single layer coating according to the modified Comparative Example 1
was provided on the 50% of the honeycomb substrate which had not been
coated with the layer coatings according to Example 2, such that 50% of
the rear section of the honeycomb substrate was coated therewith.

NOx Storage Capacity Testing

[0207] Catalytic traps were evaluated after aging for 25 hours at
800° C., as follows. An engine was set to an air/fuel ratio of
11.6 for 2 minutes at the desired temperature to remove all stored NOx
and oxygen from the catalyst. This mode represents rich engine operation.
Subsequently, the engine was adjusted to an air/fuel ratio of 29.6 under
constant NOx mass flow. This mode represents lean engine operation.
During the whole test, the NOx concentration was measured before and
after the NOx trap using a NOx analyzer.

U = NO x massoutlet NO x massinlet 100 ( 1 )
##EQU00001##

[0208] After the 2 minute rich operation followed by a lean operation
which was terminated if a concentration of 100 ppm NOx or 40 ppm NOx was
measured at the tailpipe (i.e. after the NOx trap), respectively. In that
case the engine was operated under rich conditions to remove stored NOx.
Hydrocarbon and carbon monoxide tailpipe emissions were avoided by the
HEGO sensor at the tailpipe: If the HEGO voltage increased over 450 mV,
the engine is switched to lean engine operation again. This lean/rich
cycle was repeated 10 times to establish constant catalyst conditions.
For the time period of the 10 lean/rich cycles the NOx efficiency (U) is
calculated from the NOx inlet and NOx outlet concentrations via equation
(1): NOx storage mass in g is calculated via equation (2):

[0209] After the 10 lean/rich cycles, the engine is operated for 1 min
rich to remove the stored NOx completely. Subsequently, the engine
is operated under lean condition until no more NOx is stored in the
trap. Under these conditions, the overall NOx storage capacity is
evaluated. However, to achieve a NOx conversion of greater than 80%,
the NOx storage capacity at high NOx efficiency is decisive.

[0210] As may be taken from the results, the nitrogen oxide catalyst of
the present invention displays an improved NOx storage and
conversion efficiency compared to nitrogen oxide storage catalysts of the
prior art exemplified by Comparative Example 1. In particular, as may be
taken from FIGS. 1 and 2, the application of first and second washcoat
layers, as well as the application of first and third washcoat layers
according to the present invention respectively leads to an improvement
in both the NOx storage and conversion efficiencies of a nitrogen
oxide storage catalyst, in particular at lower temperatures reflecting
the "cold start" conditions typically encountered at the beginning of the
automotive exhaust gas treatment process.

[0211] Thus, as may be taken from FIG. 1, a nitrogen oxide storage
catalyst comprising a second washcoat layer comprising Rh (Example 2) or
a third washcoat layer comprising Pd (Example 3) in addition to a first
washcoat layer comprising a nitrogen oxide storage material, respectively
leads to a clear improvement in the NOx conversion efficiency
compared to an NOx storage catalyst according to Comparative Example
1 displaying a single washcoat layer. This improvement is particularly
apparent at lower temperatures of the testing process, which reproduces
the typical "cold start" environment in automotive exhaust gas treatment.
This applies in particular with respect to Example 2, which comprises a
topcoat containing Rh.

[0212] Furthermore, as may be taken from FIG. 2, same respectively applies
for the NOx storage capacities of the NOx storage catalysts
according to Examples 2 and 3 compared to the performance of the NOx
storage catalyst of Comparative Example 1.

[0213] Most interestingly, however, as may be taken from FIGS. 3 to 6, the
nitrogen oxide storage catalyst of Example 4, which combines the NOx
storage catalyst design of Examples 2 and 3, and thus achieves a
particularly reduced total loading of Rh, surprisingly displays a higher
efficiency in both NOx conversion and NOx storage compared to
the NOx storage catalyst of Comparative Example 1 which contains
almost twice the total loading in Rh. Most importantly, said improved
efficiency is particularly pronounced at lower testing temperatures
reflecting the "cold start" environment in automotive exhaust gas
treatment.

[0214] Accordingly, as shown from the testing of Example 4, a nitrogen
oxide storage catalyst according to the present invention provides an
improved catalyst performance in automotive exhaust gas treatment with
respect to the abatement of nitrogen oxide, in particular with respect to
the critical "cold start" start conditions, in combination with a
considerable reduction of the platinum group metals necessary for its
performance, in particular with respect to the amount of Rh contained
therein.